Acrylic adhesives are more widely used than rubber adhesives in adhesive products such as optical films, protective films, labels, medical tapes, and industrial tapes using features such as good weather resistance and transparency due to the chemical structures. In general, acrylic adhesives are mainly classified into solution-type adhesives, emulsion-type adhesives, and hot-melt-type adhesives on the basis of the form thereof. Recently, ultraviolet (UV)-curable adhesives that are made to react by irradiation with UV rays after coating have also been developed.
Herein, the term “adhesive” refers to “pressure-sensitive adhesive”. The term “pressure-sensitive adhesive” refers to an adhesive which is a semi-solid having a high viscosity and a low modulus of elasticity, whose state does not change after joining, and which can be attached and detached again. The “pressure-sensitive adhesive” is usually subjected to interfacial peeling and has a high initial tack.
Solution-type adhesive/bonding agents are obtained by dissolving an adhesive/bonding component in an organic solvent. Such a solution-type adhesive/bonding agent is applied onto a base and an organic solvent is evaporated to form an adhesive/bonding agent layer on the base. Therefore, the effect of organic solvents used on the environment has been regarded as a problem. Emulsion-type adhesive/bonding agents are obtained by dispersing an adhesive/bonding component in water in the form of fine particles. Such an emulsion-type adhesive/bonding agent is applied onto a base and water is evaporated to form an adhesive/bonding agent layer on the base. Therefore, the use of a wastewater treatment and large drying energy has been regarded as a problem. Regarding UV-curable adhesive/bonding agents, for example, a liquid prepared by mixing an oligomer, a monomer, and a photoinitiator is applied onto a base, and the monomer is polymerized by irradiating the base with UV to form an adhesive/bonding agent layer. Accordingly, some of the monomer may remain, resulting in problems of the odor and toxicity. In contrast, regarding hot-melt-type adhesive/bonding agents, an adhesive component itself is applied onto a base in a thermally melted state to form an adhesive/bonding agent layer on the base. Therefore, a step of drying an organic solvent or water is not necessary, and thus such hot-melt-type adhesive/bonding agents have high productivity of adhesive products, a low environmental load, and high safety. However, since hot-melt-type adhesive/bonding agents melt when heat is applied, they have a shortcoming of a low holding power at a high temperature or under the application of a load for a long time.
Examples of reports of past examinations for obtaining an acrylic hot-melt-type adhesive that can improve the shortcoming of a low holding power include the following.
Patent literature 1 reports a hot-melt-type adhesive (pressure-sensitive adhesive composition) that uses an acrylic triblock copolymer. This report describes that since a tapered structure is not contained, the degree of freedom of the composition of the hot-melt-type adhesive is not impaired, and a cohesive strength does not decrease. However, there may be problems in terms of hot-melt processability and adhesive performance.
Patent literature 2 reports a hot-melt-type adhesive containing a particular acrylic diblock copolymer. This report describes that, by combining a particular acrylic diblock copolymer with an acrylic triblock copolymer, hot-melt processability can be enhanced while suppressing a decrease in the holding power. However, in the case where priority is given to hot-melt processability, in particular, in the case where hot-melt coating is performed at a low temperature and at a high speed, there is a room for improvement in the holding power, etc.
Patent literature 3 reports a reactive hot-melt-type bonding agent containing an isocyanate and an acrylic block copolymer. This report describes that the presence of a gradient structure between blocks decreases a green strength and increases an open time. However, since this bonding agent is a reactive bonding agent, after the bonding agent is attached to an adherend, it reacts and is bonded to the adherend, and adhesiveness is lost after the reaction. Accordingly, this bonding agent cannot be used in applications to adhesive products that are detached again, for example, tapes and labels (i.e., applications as adhesives).
Herein, the term “bonding agent” refers to an agent for which a solidification step is necessary after the agent is joined to an adherend, which exhibits a high bonding property after the joining, and which is difficult to detach again. Thus, the “bonding agent” is clearly distinguished from the “adhesive” described above. The “bonding agent” covers a “heat-sensitive bonding agent”, a reactive/UV-curable bonding agent, etc., and it is known that a bonding agent that causes cohesive failure when detached is preferable. The “bonding agent” and the “adhesive” are fundamentally different in terms of applications thereof. Using the “bonding agent” in applications of the “adhesive” is not suitable.